FIRE and MUD: Eruptions and Lahars of Mount Pinatubo, Philippines

Seismicity and Magmatic Resurgence at Mount Pinatubo in 1992

By Emmanuel G. Ramos,^{1 2} Eduardo P. Laguerta,¹ and Michael W. Hamburger²

¹Philippine Institute of Volcanology and Seismology.

²Department of Geological Sciences, Indiana University, Bloomington, IN 47405.

ABSTRACT

Seismicity at Mount Pinatubo in 1992, related to a resurgence of magma after the 1991 eruptions, can be classified into two distinct types of earthquakes and several types of volcanic tremor. High-frequency earthquakes have characteristic waveforms dominated by distinct P- and S-phases and rapidly decaying coda, similar to tectonic earthquakes. The number of high-frequency earthquakes steadily increased from the start of seismic observation at Pinatubo in April 1991 until the cataclysmic eruptions of June 1991. During the second half of 1991 and the first half of 1992, the number of high-frequency earthquakes decreased steadily from the June 1991 peak. In contrast, low-frequency earthquakes are characterized by emergent long-period waveforms and extended codas. Numerous low-frequency earthquakes preceded the cataclysmic eruption of June 1991; after the 1991 eruption, low-frequency earthquakes became infrequent and constituted only 1 in 10 recorded events.

A resurgence of low-frequency earthquakes on July 7, 1992, was followed the next day by phreatic explosions through the caldera lake and then a 4-month period of dome growth. In the later stages of dome growth, swarms of low-frequency earthquakes were common, peaking in mid-October with more than 1,400 events per day. The low-frequency events in these swarms occurred mostly as multiplets--groups of earthquakes with strikingly similar amplitude and waveform characteristics and with pronounced spatial and temporal clustering.

Volcanic tremor at Pinatubo is of three principal types: (1) low-amplitude, low-frequency harmonic tremor, observed occasionally throughout the posteruptive period; (2) large-amplitude, bimodal-frequency tremor, observed only once prior to the eruption in 1991 but frequently during the early months of 1992; and (3) high-frequency tremor that lacks distinct dominant frequencies and exhibits irregular, "spiky" envelopes. This high-frequency tremor was caused by several internal and external processes at the volcano, including lahars, secondary explosions, and eruptions.

Bimodal-frequency tremor that occurred almost daily from April to July 1992, low-frequency earthquakes that briefly reappeared in early July 1992 before the phreatic explosions, 1992 dome formation inside the 1991 caldera, and swarms of low-frequency multiplet earthquakes during late stages of dome growth are all inferred to be the result of renewed magma intrusion, 16 months after the paroxysmal eruption.

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INTRODUCTION

The work of Minakami and others (1950) at Usu Volcano, Japan, was among the first attempts to systematically classify volcanic earthquakes and to relate earthquake signal character to volcanic processes. Since then, the unusual and diverse characteristics of volcanic earthquakes and tremor have been widely documented and used to infer conditions of many active volcanoes. The 1991 eruption of Mount Pinatubo was preceded by at least 2 months of seismicity, during which earthquakes intensified in both number and size as the time of the eruption neared (Harlow and others, this volume). After the climactic eruption, the seismicity became increasingly diverse, not just because the period of observation grew longer but because of extraordinarily complex processes in the interior of the volcano. In this paper, we present an overview of the different types of seismic activity at Mount Pinatubo that focuses primarily on the events that occurred in 1992. Our preliminary classification of volcanic earthquakes is based mainly on their features on analog seismograms, supplemented by digital records. Two types of earthquakes and several classes of tremor are described, as are the possible source mechanisms of each type of seismic event.

SUMMARY OF RECENT ACTIVITY AT MOUNT PINATUBO

1990-91 ACTIVITY

To understand the seismicity at Pinatubo in 1992, it is useful to place this activity in a broader context (see papers by Harlow and others, this volume; Mori, White, and others, this volume; White and others, this volume; Punongbayan and others, this volume). Table 1 presents a brief chronology of the events at Pinatubo from the first signs of activity in July 1990 until the seismic swarms in July through October 1992. The first indication of abnormality at Mount Pinatubo was in August 1990, 2 weeks after the July 1990 Luzon earthquake (M_s 7.8), when local residents reported frequent earthquakes and "smoke" coming from the summit of the volcano (Ramos and Isada, 1990). The site was investigated but, owing to the requirements of aftershock monitoring closer to the large earthquake's epicenter, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) was unable to deploy seismic instruments at the volcano.

Table 1. Chronology of events at Mount Pinatubo, July 1990 to November 1992.

Seismic monitoring at Mount Pinatubo started in April 1991, when phreatic explosions and landslides were observed near the summit. Initial observations found that the rate of seismicity was abnormally high and that earthquake epicenters were located several kilometers northwest of the summit. The earthquakes were similar to tectonic earthquakes, with distinctly observable high frequency P- and S-phases. In the next few weeks, evidence of volcanic unrest mounted, and the cataclysmic eruptions of June 12-15, 1991, were successfully anticipated (PVO Team, 1991; Punongbayan and others, 1992; Wolfe, 1992). In addition to the high-frequency earthquakes, the other seismic events that preceded the volcanic eruption were low-frequency earthquakes and various types of tremor.

Figure 1 shows the history of seismicity associated with the 1991 eruption. The high rate of seismicity is evident in the periods immediately preceding and after the cataclysmic eruption. The seismicity rate declined rapidly after the eruption, even as explosions continued to recur at decreasing intensity. The last explosions from the crater took place in early September 1991. Most of the seismicity at that time was characterized by high-frequency earthquakes, which, by then, were gradually becoming less frequent. A shallow lake formed inside the new caldera. Some residents of villages near the volcano returned to their homes, although many still remained in evacuation centers as lahars continued to cause destruction around the slopes of the volcano.

Figure 1. Histogram of daily counts of high-frequency earthquakes at Mount Pinatubo before and after the 1991 eruption, taken from various discontinuous but overlapping analog records. Records are from Yamut (April 14 to 30, 1991), PIE (May 1 to June 15, 1991), CAB (June 16, 1991, to August 4, 1992), and PI2 (August 5 to December 31, 1992). Left-hand scale, for Yamut, PIE, and PI2; right-hand scale, for CAB.

1992 ACTIVITY

Four important events occurred at Mount Pinatubo in 1992 that interrupted the general decline of posteruptive seismicity. First, large-amplitude tremor occurred almost daily from May to July. These tremor events resembled tremor of June 4, 1991, immediately prior to the explosive eruptions. Second, a brief swarm of small-amplitude, low-frequency earthquakes abruptly appeared from July 6 to 8, 1992. Third, a new island formed within Pinatubo's caldera lake by small phreatic explosions that were too small to be recorded by the seismographs. Alert Level 3 was issued (PHIVOLCS, unpub. daily updates, 1992), and, on July 14, a lava dome began to grow (Daag, Dolan, and others, this volume), even though seismicity still remained low. The fourth important event lasted from August to October 1992, when four intense swarms of low-frequency volcanic earthquakes were recorded by the Pinatubo seismic network. Lasting from 7 to 25 days, each of these swarms contained low-frequency earthquakes of nearly uniform magnitudes and strikingly similar waveforms. The swarms peaked on October 16, 1992, when 1,422 events were recorded in 1 day, dwarfing the levels of seismicity that preceded dome formation and even that recorded before the 1991 eruption. At the time of these swarms, no significant growth of the dome was observed, and the SO₂ flux measured with the correlation spectrometer (COSPEC) showed dips in values similar to those observed before the 1991 eruption (Daag, Tubianosa, and others, this volume). Dome growth and the swarms of low-frequency earthquakes prompted PHIVOLCS to raise Alert Level 5 until December 9, 1992 (PHIVOLCS, unpub. daily updates, 1992). Concern about renewed explosive activity slowly waned as both the swarms of low-frequency earthquakes and the dome-related activity inside the crater appeared to subside.

DATA AND METHODS

In response to the developing emergency at Mount Pinatubo in early 1991, PHIVOLCS, with the subsequent assistance of the U.S. Geological Survey (USGS), deployed seismic instruments to a total of 23 different sites around the volcano. Analog portable seismographs (Kinemetrics Ranger and Sprengnether MEQ 800 Seismographs with 1-s Ranger seismometers) recording on smoked paper were the first instruments deployed. When the high rate of seismicity was confirmed, digital event data recorders (Kenkei Systems EDR 1000 with 1-s L2 seismometer) were also deployed. All of these instruments were operating independently until late April 1991, when a telemetered, PC-based seismic monitoring system was installed by the USGS to centralize the system and facilitate data analysis. The seismometers used in the telemetered system were 1-s natural period Mark Products L4 vertical and horizontal sensors (Lockhart and others, this volume; Murray and others, this volume). Murray and others (this volume) provide a summary of the PC-based data processing. Routine data analysis included manual phase picking for hypocentral locations and measuring coda amplitudes and durations for magnitude determinations. The analog data were collected from the same network by tapping the input lines to the digital data acquisition system and recording on pen-ink drum recorders. Only one of the original seismic stations (CAB) survived the eruption; after the eruption, CAB and one other original station (BUG) were repaired, and three new stations (PI2, FNG, and QAD) were added. In August 1991, recording on portable seismographs was discontinued, leaving only the telemetered, PC-based system in operation. In August 1992, a new seismic station (CRA) was installed near the rim of the 1991 caldera (fig. 2).

Figure 2. Location of telemetered seismic stations at Mount Pinatubo. Most preeruption sites of analog recorders were located farther from the summit, outside of the boundaries of this map. Open triangles are stations destroyed by the eruption, and closed triangles are stations operating in 1992. Signals were initially transmitted to CAB until early 1992, after which time they were transmitted to the PHIVOLCS office in Quezon City. Contour lines of 500- and 1,000-m elevations show preeruption topography. Hatched circle marks 1991 caldera.

In our analysis of the earthquakes, we have made extensive use of the analog records obtained from the telemetered network. The completeness and continuity of these records are especially useful for studying small-magnitude events and those with emergent onset that do not ordinarily trigger the digital network. The analog records are also useful for studying temporal relations between events, because they show a continuous data stream. There are also significant disadvantages in using these analog records, however. These disadvantages include (1) inaccuracy in the timing of the analog seismograms, since the analog recorders relied on independent timing systems that were not synchronized with the central digital system; (2) difficulty in making spectral measurements, particularly for high-frequency parts of the records; (3) limitation on the dynamic range of amplitude recorded on the analog seismograms, constrained by the physical limits of the pen-ink recording system which thus made it impossible to measure actual amplitudes of large events; (4) limitations on the number of analog recorders connected to the digital system (only two stations were connected consistently to analog recorders before July 1992, when more analog recorders became available, so station coverage of the analog records is inferior to that of digital records); and (5) the unavailability of calibration data for the analog recorders, which prevented us from using these records for magnitude calculations. Analog seismograms from stations PIE and UBO for the preeruption period (May-June 1991), and seismograms from station PI2 for the period from January to August 1992, were used to classify events and to measure phase arrival time, amplitude, and coda duration. All events exceeding 4 mm in amplitude on recording charts were noted; spectral frequencies of tremor were measured with an optical loupe.

Earthquake hypocentral locations obtained from routine digital data analysis were used to define the spatial distribution of the events. Coda-magnitude determinations from the digital seismograms were available for events detected by the digital system. Spectral analysis of the digital records of the two types of earthquakes was accomplished by using the International Association of Seismology and Physics of the Earth's Interior's (IASPEI's) program PCEQ (Valdez, 1989).

Analysis of the volcanic tremor presented an unusual difficulty, since none of these events were recorded in the digital system. Our analysis was entirely dependent on 1992 analog records that were available only for two stations, PI2 and FNG (fig. 2), for most of the study period. For tremor events preceding the 1991 eruption, we have also referred to the seismograms gathered by the portable analog seismographs deployed around the volcano.

CLASSIFICATION OF SEISMIC EVENTS AT MOUNT PINATUBO

The seismic activity at Mount Pinatubo is of two distinct varieties: discrete short-duration earthquakes and long, usually emergent, slowly decaying volcanic tremor. The earthquakes can be classified further into high-frequency and low-frequency events, according to the dominant frequency content of the signal. We also have classified tremor according to frequency, namely: low frequency (monochromatic or harmonic), bimodal frequency, and high frequency. High-frequency tremor was classified further into genetic subdivisions.

HIGH-FREQUENCY EARTHQUAKES

Waveform description.--The most common type of seismicity at Mount Pinatubo is the high-frequency (short-period) earthquake (fig. 3). The waveform of these events resembles that of typical tectonic earthquakes recorded near the source. The signature is dominated by distinct body wave P- and S-phases. The high-frequency type corresponds to Minakami's "A-type" (Minakami and others, 1950; Minakami, 1974), to the "volcano-tectonic earthquakes" of Latter (1979), and to the "type h" of Malone (1983). The onset of the high-frequency earthquakes is often very impulsive, although emergent arrivals that are occasionally observed at individual stations may be due to attenuation of the P-phases along raypaths traversing or skirting magma chambers (Latter, 1984) or possibly to a particular orientation of the energy radiation pattern at the earthquake source. Figure 4A shows a seismogram of a high-frequency earthquake and the corresponding displacement spectrum of the signal. The peak of the spectra for these high-frequency events tends be nearly flat between 1 and about 6 Hz.

Figure 3. Seismograms from PIE (from PI2 after the 1991 eruption) showing high-frequency earthquakes of various size and from different hypocentral locations, as recorded in May 1991 (A) and May 1992 (B). B shows three other types of events, a tectonic earthquake (Te), harmonic tremor (Th), and bimodal-frequency tremor (Tb). Time runs from top to bottom, left to right. Time marks are 60 s apart. Date and time shown are local.

Figure 4. A, Seismograms of a typical high-frequency earthquake at Mount Pinatubo, May 15, 1991, as recorded at stations UBO and PIE. Amplitudes are normalized for comparison of waveforms. Time marks in seconds are shown below each seismogram. B, Instrument-corrected amplitude spectrum from the UBO signal for the same event as in A. Note the broad, relatively flat spectral signature. C, Seismograms of a typical low-frequency earthquake at Mount Pinatubo, August 19, 1992, as recorded at stations CRA and PI2. D, Instrument-corrected amplitude spectrum from the CRA signal for the event shown in C. All records are from vertical seismometers, hence the Z suffix on station names.

Like tectonic earthquakes, the waveform of high-frequency earthquakes is also dependent on magnitude, depth of hypocenter, and source-to-station distance. High-frequency earthquakes of larger magnitude, shallow depth, or distant sources tend to be dominated by lower frequencies because of high-frequency attenuation and (or) the generation of surface waves. Events classified by Minakami and others (1950) as "B-type," and the "m-type" events at Mount St. Helens (Malone, 1983), may reflect these variations in the high-frequency earthquakes (Hamada and others, 1976; Chouet, 1988).

Space-time distribution.--The high-frequency earthquakes have a wide range of spatial, temporal, and magnitude distribution. High-frequency earthquakes include the cluster of events prior to the 1991 eruption that were located to the northwest of the volcano at depths of about 4 to 10 km and include the more extensive posteruptive seismicity that ranged in depth from the surface to about 18 km (Mori, White, and others, this volume). The lateral distribution of high-frequency earthquakes is also wide, extending well beyond the edifice of Mount Pinatubo proper. The northwest cluster of preeruptive seismicity and the more extensive seismicity after the eruption consist mostly (>80%) of high-frequency earthquakes (Mori and others, 1991).

The number of high-frequency earthquakes increased sharply as the 1991 eruption neared and then decayed rapidly afterwards. In the first half of 1992, the high-frequency earthquakes continued their slow decrease (fig. 1). Prior to dome formation in 1992, the number and magnitude of high-frequency events changed. A brief and subtle swarm occurred from June 20 to 22 (fig. 5, top). A noticeable decrease in both number of events and magnitude followed, during the period of about 8 days starting around June 23, 1992. The level of high-frequency seismicity again increased before a short swarm of low-frequency seismicity and the dome formation in July 1992.

Figure 5. Duration and maximum amplitude of earthquake codas, and maximum amplitude of bimodal-frequency and harmonic tremor, as recorded at station PI2 before the dome-forming episode in 1992. All counted earthquake codas were >=4 mm in amplitude on PI2's analog record. All tremor events with amplitudes >=20 mm are bimodal-frequency tremor. Clipping amplitude was 48 mm. Note the reduced number and size of both high- and low-frequency earthquakes prior to the July 7-8 swarm of low-frequency events and the dome formation.

Magnitude distribution.--The magnitude distribution (based on coda measurements) of high-frequency earthquakes for the 16 months from April 1991 to August 1992 shows that the events ranged in size from magnitude M_L=0 to greater than M_L=4 (>M_L 5.5 on June 15, 1991). This broad magnitude spectrum is observed for both long and short time duration. Figure 5 illustrates the wide magnitude distribution of high-frequency events over the entire study period; figure 3 shows a similarly wide range of earthquake sizes over the one-day period of a single seismogram. Mori, White, and others (this volume) calculated a b-value (Richter, 1958) of 1.1+-0.3 for this type of Pinatubo earthquakes.

LOW-FREQUENCY EARTHQUAKES

Waveform description.--The second type of discrete event observed at Pinatubo is the low-frequency earthquake (fig. 6). Harlow and others (this volume) and Murray and others (this volume) use the term long-period earthquake for this type of event at Pinatubo in 1991. The low frequency signal peaks at about 1 Hz (fig. 4B), and the onset of these earthquakes is usually emergent and lacks distinct P- and S-phases. These events tend to have long coda duration with respect to their amplitudes, adding to their distinction from high-frequency earthquakes. The typical waveform of low-frequency events shows some pulses of energy (or pseudophases) that seem to persist from one event to another. These pseudophases are not easily correlatable between stations, and their overall characteristics seem to differ from those of typical P- and S-phases, being of low frequency and lacking the pulse of energy associated with the arrival of direct body phases. On some large events, particularly on seismograms recorded near the crater (station CRA), the early part shows as an impulsive phase, although even this is usually preceded by an emergent, low-amplitude, high-frequency signal resembling a refracted phase arrival. A high-frequency component is often observed early in the low-frequency earthquake signal, similar to that described by Fehler and Chouet (1982) and Chouet (1992) at other volcanoes. Clearly, some high-frequency content is present in the earthquake source and is released in the early part of the event.

Figure 6. Seismograms re- corded at station FNG during the swarms of low-frequency earthquakes in (A) September 1, 1992, (B) October 8, 1992, and (C) October 24, 1992. Instrument setting was the same for all records. Note uniform size of the earthquakes in each sequence. The similarity of the waveforms, most noticeable in C, indicates that many of these events are multiplets. Time marks as in figure 3.

Space-time distribution.--Unusual, deep, low-frequency events occurred on May 26 and June 4, 1991 (White, this volume). In the succeeding week, low-frequency events were overshadowed by shallow high-frequency events; then, during the period of explosive eruptions between June 12 and 15, shallower low-frequency events became prominent (Harlow and others, this volume). During the remainder of 1991, high-frequency earthquakes were about 10 times as numerous as low-frequency earthquakes.

In 1992, low-frequency earthquakes had an unusual temporal distribution. Low-frequency events began in early May and then became markedly absent for a period of about 8 days, from June 24 to about July 2, coincident with reduced high-frequency seismicity (fig. 5). On July 7 and 8, immediately preceding the phreatic explosions and the start of dome formation, a small swarm of low-frequency earthquakes occurred. The rate of occurrence of low-frequency earthquakes was slightly elevated, averaging about 15 per day, when, on July 14, the dome began to grow. This number of events gradually increased by the end of July, developing into a swarm with about 120 events per day. This was the first of four intense swarms recorded in 1992, each swarm lasting from 7 to 25 days (fig. 7). The peak number of events in each swarm also increased. In the last swarm, on October 24, the highest number of earthquakes in the history of Mount Pinatubo was recorded when 1,422 events were recorded in a day.

Figure 7. Histogram of high-frequency (foreground) and low-frequency earthquakes recorded at station FNG from June to December 1992.

Hypocenters of the low-frequency earthquakes were within a relatively small region, mostly near or beneath the caldera, usually within a radius of 2 km from the growing dome. The events were always shallow, usually between 0 and 3 km depth, and rarely deeper than 5 km (fig. 8).

Figure 8. Map of low-frequency earthquakes in August and September 1992. Most of the better located events cluster around the crater, have shallow depths, and have magnitudes (MAGN.) less than 1.

Magnitude distribution.--The range of magnitudes of low-frequency earthquakes is much narrower than that of high-frequency earthquakes. Although some low-frequency events in 1991 were recorded as far as 300 km away (PVO Team, 1991), the majority of 1992 events did not exceed a magnitude of 1. These earthquakes tend to occur as groups of earthquakes with uniform sizes, as illustrated by the seismograms in figure 6. In the August-October 1992 swarms, the low-frequency earthquakes changed size in unison, from one cluster of earthquakes to the next, with each size being retained only for a few hours to a few days. The b-value we have calculated for low-frequency earthquakes is 1.46+-0.04.

Earthquake multiplets.--One very unusual feature of the recent swarms of low-frequency earthquakes is the uniformity of their waveform. These earthquakes occurred as multiplets--earthquakes with very similar signatures. Figure 9 shows the first 30 s of the low-frequency earthquakes recorded at stations PI2 and FNG in the first days of the August 1992 swarm. These events were randomly picked from the hundreds of earthquakes that occurred that day, and virtually all of them showed strikingly similar waveforms. From the start to the end of their codas, the signature of the earthquakes is almost identical, with peaks and troughs matching from one event to the other. Multiplets have been noted in many other sites, including Usu Volcano, Japan (Minakami and others, 1950; Okada and others, 1981), Mount St. Helens, United States (Fremont and Malone, 1987; Neri and Malone, 1989), Ruapehu Volcano, New Zealand (Dibble, 1974), and sites away from volcanoes (for example, Cole and Ellsworth, 1992; Sherburn, 1992). Okada and others (1981) termed events with such behavior as belonging to "earthquakes families." Minakami and others (1950) described events at Usu Volcano with similar repetition of waveforms and classified the events as "type C." The similarity in waveform of multiplets indicates that the events occur within a very limited region of space (Dibble, 1974; Fremont and Malone, 1987; Neri and Malone, 1989; Cole and Ellsworth, 1992) and that the earthquakes share the same source mechanism (Fremont and Malone, 1987). A significant feature of low-frequency earthquake multiplets in volcanoes is that they frequently precede or accompany dome formation (Fremont and Malone, 1987; S.D. Malone, written commun., 1993).

Figure 9. Seismograms of the first 30 s of multiplet low-frequency earthquakes on September 3,1992, randomly chosen from the hundreds of events in ongoing swarm. Only FNG and PI2 traces are shown, although all stations recorded the events with identical signature from onset to the end. A high-frequency earthquake of September 7 is shown for comparison.

VOLCANIC TREMOR

Volcanic tremor, in contrast to the earthquakes, lasts for extended periods of time (Malone and Qamar, 1984); at Mount Pinatubo, tremor lasted from a few minutes to a few days. Tremor at Mount Pinatubo is classified into low-frequency (harmonic), bimodal-frequency, or a broad category of high-frequency tremor.

HARMONIC TREMOR

The type of tremor with the narrowest frequency spectra is the harmonic tremor. Every event of this tremor is monochromatic, occurring with a very characteristic narrow frequency that remains constant throughout the duration of the events (fig. 10A,B). The dominant frequency may vary from event to event, but the range of frequency is still narrow, from 0.7 to 2.4 Hz. One feature of this tremor is its pulsed amplitude that results in regularly spaced lobate envelopes of uneven sizes. This tremor is emergent, initially appearing from the background as small pulses and gradually emerging as symmetrical, smoothly formed trains of lobes, then gradually fading into the background. The duration of harmonic tremor is highly variable and is inversely proportional to its amplitude. The large-amplitude harmonic tremor may be as short as a few minutes, while the small-amplitude events may last for a few hours to a few days, remaining as background microseism while other events occur. The longest period of harmonic tremor recorded at Pinatubo began on June 5, 1992, when a continuous background tremor was visible on the record for 72 h. In some events, the tremor repeatedly occurs after brief periods of repose, creating contrasting dark and light bands on the seismogram that are similar to those described by McKee and others (1981). In 1992, the harmonic tremor seemed to be most frequent in the 2 months preceding the dome formation (fig. 5). A crude estimate of tremor source depths can be made by comparing the relative amplitudes of tremor at various stations to the relative amplitudes of well-located earthquake signals at the same stations. Using this procedure, we judge that some of the harmonic tremor originates between 5 and 10 km below the caldera.

Figure 10. Seismograms of (A, B), harmonic tremor, (C), a lahar signal, and (D), high-frequency tremor. The signal on upper part of B is the tail of some bimodal-frequency tremor. In C, the pen was manually offset twice, as shown by arrows, to avoid overprinting of one rotation's signal on the next. All of the large-amplitude signal is of lahar. In D, the high-frequency tremor at station UBO, recorded prior to dome growth in 1991, faded to the background 2 h after the segment shown. Time marks as in figure 3.

BIMODAL-FREQUENCY TREMOR

Another type of tremor recorded at Pinatubo has waveforms characterized by the superimposition of two signals with two distinctly different frequencies, one high and the other low (fig. 11). The high-frequency portions of the tremor is always of low amplitude and usually starts earlier and ends later than the low-frequency segment, forming a "head" and a "tail" of the event (fig. 12). The peak frequency in the head and tail portions is relatively high, at about 2.4 Hz. This high-frequency signal appears to persist throughout the duration but is overridden by the low-frequency component in the central part of the tremor; it is only visible when the amplitude of the low-frequency portion subsides. Near the end of the signal, the low-frequency portion ends, and again, the high-frequency signal dominates until finally fading into the background. The low-amplitude high frequency head starts an average of 1.6 min (sometimes, 25 min) earlier and the high frequency tail finishes 1 to 11 min later than the low-frequency body, in proportion to the total duration of the whole tremor. Both the head and the tail portions gradually emerge from and fade to the background. Maximum amplitude of the head and tail average about one-tenth of the large-amplitude low-frequency signals.

Figure 11. Seismograms of portions of bimodal-frequency tremor. A, Sustained bimodal-frequency tremor about 2120 on May 17, 1992, as recorded at PI2. B, C, Records of a short-lived bimodal-frequency tremor as recorded at two stations, PI2 and FNG. Record in C covers a subset of the time represented in B. The high-frequency portion of the tremor is more visible in the early or low-amplitude portions of the tremor. Time marks as in figure 3.

Figure 12. Seismograms of bimodal frequency tremor events in May and June 1992, all recorded at station PI2. Note that the short-duration, large-amplitude low frequency sections are preceded and trailed by long-duration, low-amplitude, high frequency signals. Time marks are 1 min apart and the seismograms cover almost the whole length of the 10-min-wide helicorder chart. The recording pen was manually shifted while writing seismogram 12C to reduce overlapping of signals.

The low-frequency segment (or body) of this bimodal-frequency tremor is usually monochromatic and emerges from the high-frequency head (figs. 12A,B). The dominant frequency is constant in all events at about 1.0 Hz. The amplitude is usually large, often clipping at PI2, and, as such, is much more vigorous than the earlier mentioned harmonic tremor. The body is composed of sinusoids of smoothly but constantly varying peaks that form into pulsed amplitude-symmetric envelopes. Compared to the harmonic tremor, the bimodal-frequency is less monochromatic, exhibiting less symmetry along the time axis (that is, the trailing part does not mirror the leading part), and may be caused by the presence of high-frequency components in the tremor. The duration of the low-frequency body may be as short as 30 s and as long as 22 min, generally proportional to the total length of the tremor. On some bimodal events with low amplitude, the low-frequency portions appear to resemble harmonic tremor, but the presence of the high-frequency component in bimodal-frequency tremor distinguishes these from the monochromatic tremor (compare the top and bottom parts of fig. 10B, for example). The overall duration of the bimodal-frequency tremor, including the high-frequency head and tail, ranges from 90 s to 112 min.

Pinatubo's bimodal-frequency tremor usually precedes low-frequency earthquakes. This tremor may have appeared before the eruption, on June 4, 1991 (White, this volume), but was not observed during the eruption or in the next 11 months. On May 3, 1992, it began to occur almost daily until July 19, 1992. Despite their large amplitudes, the emergent nature of these events prevented them from being detected by the digital system and thus are only recorded on the analog records.

Figure 11B and C show seismograms of a bimodal-frequency tremor recorded at stations PI2 and FNG. Characteristic of the tremor shown by these seismograms is the similar frequency content of the signal recorded at the two stations and the slight resemblance of the tremor envelope recorded at the two stations. This consistency in the frequency and the resemblance of waveform between stations suggest that these features are caused at the source and not by the wavepath (Aki and Koyanagi, 1981). We infer the probable source region for the bimodal-frequency tremor to be deep, probably greater than the half-aperture of the seismic network, in order for these events to preserve not only their frequency but also their waveform as the waves travel to the various seismic stations. This interpretation is consistent with the observation of White and others (this volume) of deep, long-period earthquakes or tremor on June 4, 1991. Tremor amplitudes on two analog records indicate at least two probable sources for these tremor, one source generating signals that are consistently larger at FNG than PI2 (average ratio 1:2) and another generating larger signals at PI2 than FNG (average ratio 1:4). Initial estimates of source depths, based on comparisons with located high-frequency earthquakes, suggest that tremor with amplitudes largest at PI2 originates between 4 and 8 km in depth. The absence of any known surface activity at Pinatubo's new crater that might correlate with the occurrence of this type of tremor (fig. 11) further supports this inference of a nonsurface source for the bimodal-frequency tremor.

HIGH-FREQUENCY TREMOR

Other tremor at Mount Pinatubo does not exhibit distinct dominant frequencies and has higher and wider frequency content than the harmonic and bimodal-frequency types. Most is spasmodic, composed of randomly distributed peaks without any well-formed envelopes. Amplitude can increase or decrease abruptly, unlike smooth changes in amplitude in the two previous types of tremor. Much of this high-frequency tremor is genetically associated with lahars, steam emission, and explosive venting of ash. In detail, we recognize the following three subtypes of high-frequency tremor.

Lahar-induced tremor.--Like the head of bimodal-frequency tremor, the onset of lahar-induced tremor is an emergent, high-frequency signal (fig. 10C). The amplitude of the lahar signal grows more slowly and more steadily than does the amplitude of bimodal-frequency tremor, and high-amplitude portions tend to contain especially high-frequency content. Some short portions of lahar signals appear to be low-frequency surface waves generated by lahar flow or signals from secondary phreatic explosions (described below). Episodes of lahar-induced tremor usually last from about 20 min to a few hours.

Explosion events.--Explosions from Pinatubo's crater usually contain substantial energy and produce high-frequency signals that reach most stations. Figure 13A shows the seismic trace of an early, small, phreatic explosion in May 1991 from a vigorously steaming fumarole on the north flank of Mount Pinatubo, and figure 13B shows seismicity before and at the start of large explosions of June 12, 1991.

Figure 13. Seismograms of explosions from (A, B) Mount Pinatubo's crater and (C) 1991 pyroclastic deposits. A is part of a series of phreatic explosions from the 1991 pre-eruption summit vents. B shows the microseisms, earthquakes, and the start of preliminary explosions in June 1991, prior to the cataclysmic eruption. C is from a large phreatic explosion on a pyroclastic deposit accompanying a small lahar in the Sacobia River.

Explosions in hot, 1991 pyroclastic-flow deposits are also sources of tremorlike signals. Secondary explosions are caused by near-surface interaction of water and hot pyroclastic materials. These explosions can generate surface waves that appear as low-frequency pulses on seismograms (figs. 10C, 13C). These low-frequency pulses are brief and are recorded only at stations close to the source. Unlike bimodal-frequency tremor, signals from secondary explosions are neither monochromatic nor contained within a smooth amplitude envelope. Tremor from these explosions often blends with longer, lahar-generated tremor, but, because water percolation into the pyroclastic deposits may be delayed or be independent of lahars, secondary explosions can also occur on their own (fig. 13C).

Shallow high-frequency tremor.--On June 6, 1991, shortly before the preclimactic dome began to grow, very high-frequency, low-amplitude tremor started to occur in the seismic records of the near-summit station UBO. This tremor was emergent and gradually increased (fig. 10D), coincident with increased steam and ash emission from the most vigorous vents. The fact that this tremor was not recorded at stations farther away from the crater indicates that the source of this tremor was probably shallow.

SEISMIC SOURCE MECHANISMS OF THE 1992 EVENTS

A summary of the frequency and coda duration characteristics of the various types of seismic events at Pinatubo is presented in figure 14. Significant overlap in the frequency and coda duration of the various events suggests that a classification scheme based entirely on these two characteristics would be insufficient to completely separate different classes of events. Table 2, a summary of the types of seismic events observed at Mount Pinatubo, includes additional parameters for improved discrimination between event types. In table 2, we indicate the degree to which observable seismologic features of various events are common to seismograms recorded at different seismic stations, and we indicate the degree to which seismograms of a single event resemble earlier or later events of the same type. In general, events with high interstation similarity may be assumed to occur at greater depth, because these signals arrive at a steep angle and are less strongly affected by the raypath and local station conditions. High-frequency earthquakes and the low-frequency (harmonic) tremor are examples of these events. Events with high interevent similarity may originate from limited regions of space and may have easily repeatable source processes. Lahars and multiplet low-frequency earthquakes exhibit these features.

Figure 14. Duration-frequency distribution of various types of Pinatubo's volcanic seismicity. Data on tremors are from frequency counts using optical loupe on analog records, and those for earthquakes are from the Fast Fourier Transform of digital earthquake seismograms.

Table 2. Summary of waveform, amplitude, spatial, and temporal characteristics of Mount Pinatubo's earthquakes and tremor.

[Near-surface events have low interstation but high interevent correlation]

¹Spectral character of tremor was determined from analog seismograms.

High-frequency earthquakes, being similar to tectonic earthquakes in many ways, probably represent shear failure on a fault or fracture under stress (Weaver and others, 1981; Malone, 1983). Frequent earthquakes after the 1991 eruption were vertically and laterally more extensive than the preeruptive seismicity (Mori, White, and others, this volume) and were probably along local and regional faults, in response to deflation of the magma chamber and loading by new pyroclastic deposits on the surface.

Low-frequency (or "long-period") earthquakes can be generated by magma and related fluids ascending through existing channels (Shaw and Chouet, 1989; Chouet and others, 1994) or intruding into competent rocks (Latter, 1984). The shallow depths of most low-frequency events at Pinatubo, the existence of high-frequency signals at their onset, and their occurrence prior to surface magmatic activity corroborate the involvement of magmatic fluids in their generation. Multiplet low-frequency earthquakes at Mount Pinatubo during the last months of 1992 suggest that the earthquake source was in a very limited volume and that the earthquake-generating process was easily repeated. Multiplet low-frequency earthquakes can, therefore, be interpreted as repeated, shallow-level intrusions of magma.

We have classified volcanic tremor at Pinatubo according to its characteristics on seismograms recorded at various stations. On the basis of different waveform characteristics at different stations, we infer that the source mechanisms and possibly the source regions for the bimodal-frequency tremor are different from those of low-frequency (harmonic) tremor. A nonsurface origin of the bimodal-frequency events is inferred from (1) uniformity of the frequency and the similarity of its waveform envelopes at the various recording stations; (2) similarity of its waveform features with the low-frequency earthquakes, which often had source depths between 0 and 7 km; (3) their occurrence before the dome extrusion and its associated seismicity; and (4) the absence of correlatable activity at the crater at the time of these tremor. We tentatively infer that the source for bimodal-frequency tremor is deeper than that of the low-frequency earthquakes because the former usually precedes the latter. Thus, we interpret the bimodal-frequency tremor to be related to magmatic movements at depth, possibly during its ascent to shallower levels.

Low-frequency (harmonic) tremor at Mount Pinatubo may be caused by a different mechanism. The low-frequency content of the harmonic tremor partly overlaps that of the low-frequency earthquakes and of bimodal-frequency tremor (fig. 14). Many of the seismograms we have reviewed contained all three events adjacent to one another. The events, although sometimes occurring at similar amplitudes, nonetheless remained distinguishable from one another and preserved their own distinct waveforms. The consistent absence of high-frequency signals in Pinatubo's harmonic tremor, and consistently low amplitude of harmonic tremor (<20 mm at PI2; fig. 5), suggests that it is generated by a process different from that which causes the low-frequency earthquakes and the bimodal-frequency tremor. From the monochromatic waveform, we infer that the source of the harmonic tremor can be some form of resonance of a magmatic conduit of some geometry. The requirements for producing such resonance are the presence of a triggering transient that initiates the vibration (Ferrick and St. Lawrence, 1984; Chouet, 1988) and some means of sustaining a vibration for extended duration (Steinberg and Steinberg, 1975; Ukawa and Ohtake, 1987; Chouet, 1988; Shaw and Chouet, 1989). The transients to trigger the event can be provided by Pinatubo's high-frequency earthquakes or some energetic pulses in magmatic movement. The geometrical and material elasticity requirements needed to sustain the resonance can be provided by the volcano's plumbing system. The similarity of the frequency and waveform of these earthquakes with volcanic tremor allows volcanic tremor to be considered as repeated occurrences of low-frequency earthquakes and the earthquakes to be considered as the impulse response of the tremor-generating process (Shaw and Chouet, 1989; Chouet and others, in press).

A schematic illustration of the spatial interrelation of the various seismic and volcanic processes at Mount Pinatubo is shown in figure 15.

Figure 15. Schematic illustration of the spatial interrelation of Pinatubo's seismicity and volcanic processes.

IMPLICATIONS OF THE 1992 EVENTS

We expected that the 1991 eruption of Mount Pinatubo would be followed by dome formation because previous eruptions of Pinatubo had culminated in dome growth (Newhall and others, this volume), as had explosive eruptions at Mount St. Helens (Fremont and Malone, 1987) and Usu (Okada and others, 1981). No dome formed immediately after Pinatubo's 1991 explosive events, but a dome did form 1 year later, in mid-1992, after repeated occurrence of bimodal-frequency tremor. We associate these bimodal-frequency tremor events with renewed magmatic intrusions because of their waveform, timing, and inferred depth. A fundamental question, then, is whether the dome that formed in 1992 represents the culminating phase of Mount Pinatubo's 1991 eruption or, alternatively, an early phase of further eruptions.

Two models can be constructed from the recent chronology of events. The first assumes that the swarms of low-frequency multiplet earthquakes in 1992 represent shallow-level intrusion of magma, after a deeper level influx of new magmatic material that was marked by bimodal-frequency tremor. In this scenario, the 1992 dome formed by fresh intrusion of basaltic magma into residual 1991 dacitic magma, mixing of basaltic and dacitic magma, and ascent of the mixture to the surface, similar to events that occurred in April through June 1991. In 1991, ascent of the dome-forming mixture triggered explosive eruption of volatile-rich dacitic magma (Pallister and others, 1992; Pallister and others, this volume). Volatiles in the remaining dacitic magma may have been so depleted after the 1991 eruption that renewed intrusion in 1992 produced another dome but could not trigger renewed explosive eruptions. In this model, activity in 1992 represents a dying phase of the cataclysmic eruption of 1991, and renewed explosive eruptions are considered unlikely in the near future.

An alternative model, featuring similar intrusions, is that following the 1991 eruption, the presence of a low-confining-pressure, relatively open-vent environment may have eased the ascent of magma to shallow levels beneath the volcano. The formation of the dome in 1992 may have capped this low-pressure system, and continuing low-frequency earthquakes and volcanic tremor, which have continued intermittently through early 1994, may represent ongoing intrusion and pressurization of the shallow-level magmatic plumbing system. If this model proves correct, further explosive activity remains possible. More detailed study, and further events themselves, will reveal which is the better model.

CONCLUSIONS

We have classified 1991 and 1992 seismic events at Mount Pinatubo into two types of earthquakes and several types of tremor, summarized as follows:

1. High-frequency (volcano-tectonic) earthquakes: tetonic-like events that are related to structural adjustments of the crust and volcanic edifice, especially during heightened volcanic activity.

2. Low-frequency (long-period) earthquakes: events with long-period signals that likely are associated with magmatic intrusion. Swarms of multiplet low-frequency earthquakes indicate that the source process of the low-frequency earthquakes is easily repeatable.

3. Low-frequency (harmonic) tremor: monochromatic signals that probably are generated by the resonance within the magmatic plumbing system of Pinatubo.

4. Bimodal-frequency tremor: tremor of mixed high- and low-frequency signals that are probably associated with magmatic transport below the source region of low-frequency earthquakes.

5. High-frequency tremor: high-frequency events with a variety of possible sources that may or may not be directly related to magmatic activity. The various sources include lahars, explosions from vents and from fresh pyroclastic deposits, and degassing from active fumaroles.

Our classification of seismic events at Mount Pinatubo is preliminary and will require additional refinements as the immense volume of data available from Pinatubo is scrutinized in greater detail. In the process of trying to relate features of volcanic tremor from Pinatubo to those of other volcanoes, we encountered considerable uncertainty in comparing waveform descriptions, either because the available literature does not provide enough detail about the waveforms or because very few seismograms of tremor have been published. In general, our classification of volcanic earthquakes and tremor into various genetic types requires that the events associated with the seismicity be confirmed by other forms of observation, a requirement that can only be met under ideal observational conditions.

ACKNOWLEDGMENTS

The data we have used were gathered by the collective efforts of the PHIVOLCS and USGS personnel involved in the operations of the Pinatubo Volcano Observatory. The USGS provided the PC-based seismic instruments during the 1991 crisis, and the support of the USAID's Office of Foreign Disaster Assistance made it possible for PHIVOLCS to retain the system. The continued involvement and interest of USGS personnel, particularly Chris Newhall, Dave Harlow, Jim Mori, Paul Okubo, and John Power, were helpful in providing insight into the volcanic and seismic activity. Steve Malone of the University of Washington guided us through his work at Mount St. Helens, particularly his studies of multiplets and long-period earthquakes and tremor. Ray Punongbayan provided encouraging support, and the National Science Foundation provided funds, under NSF grant No. EAR-9121566, which made this research possible. Constructive reviews by Steve Malone, Motoo Ukawa, and Chris Newhall greatly improved our appreciation of events described in this paper.

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